The present invention relates to a safety valve for a compressed-gas tank, the said valve comprising a plug made of meltable material and intended to melt at a predetermined temperature in order to release the compressed gas from the tank.
Such safety valves are used to prevent the risk of the tank exploding when the temperature exceeds a predetermined limit. In fact, at this temperature, which may be of the order of 100xc2x0 C., the plug mentioned in the introduction melts and allows the gases and excess pressure to escape. Below this critical temperature, the plug must normally remain solid and prevent any leakage of pressure and gas.
Unfortunately, the situation was found where the plug melts prematurely, that is to say at temperatures below its melting temperature. It was found that these instances occur when the gas is stored at high pressures and when this melting is caused not by the temperature, but by the pressure. It was discovered, in fact, that if such a plug is exposed to high pressures for a relatively long time, it may become deformed and even melt and thus release the gases below the melting temperature. There is therefore an unintentional leakage, along with the risk of an accident and risks to the environment. Moreover, the material of the plug to be replaced is extremely costly.
The object of the present invention is to provide a new safety valve which makes it possible to avoid these disadvantages, that is to say a valve, the plug of which does not risk melting prematurely under the effect of prolonged pressure.
In order to achieve this object, the invention provides a safety valve of the type described in the introduction, which is essentially characterized in that the meltable plug is subjected to the action of a differential-action sliding piston intended for. educing at least some of the effect of the gas pressure on the meltable plug.
According to a preferred embodiment, the valve comprises a housing integral with the tank and comprising a first cylindrical part open towards the outside of the tank and a second cylindrical part with a closed bottom, having a diameter smaller than that of the first cylindrical part and communicating through its side wall with the interior of the tank, and a bush which is fastened inside the first cylindrical part and the orifice of which faces the second cylindrical part, the bush containing the meltable plug which is retained there by means of a rod integral with the said differential-action piston which is located in the said second cylindrical part. The said differential-action piston is preferably a double-action piston comprising two pistons connected by means of an axial rod, the first piston normally being located at the bottom of the second cylindrical part and the second piston normally being located between the first and the second cylindrical part in order to ensure sealing between these parts, the diameter of the latter piston being greater than the diameter of the first piston.
The effect of the gas pressure on the second piston consequently generates a thrust on the meltable plug, the said thrust being proportional to the pressure of the gas and to the exposed surface of the second piston. By contrast, the first piston, which is likewise exposed to the pressure of the gas, acts in the opposite direction, that is to say its effect is subtracted from the thrust exerted by the second piston on the meltable plug. Finally, the latter undergoes only a force proportional to the difference in the areas of the two pistons. The plug consequently undergoes only a slight pressure and does not risk being damaged prematurely under the effect of a high and prolonged pressure.